Hello everyone, hope you are doing well.
I work in control systems sometimes and I was wondering if you come to a circuit and you know that it is most likely either 0-10vdc, or 4-20mA, how do you find out which one it is? Note: the voltage across the two conductor circuit is less than one volt.
Thank you for your time
Skinned
Dont let the name scare you.

Hello everyone, hope you are doing well.
I work in control systems sometimes and I was wondering if you come to a circuit and you know that it is most likely either 0-10vdc, or 4-20mA, how do you find out which one it is? Note: the voltage across the two conductor circuit is less than one volt.

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Only possible if a slight disturbance can be tolerated by the device under control.

If so, the following circuit can be used. Basically you apply a load to the control voltage/current.

If the voltage doesn't change with the press of the push button, then it would most likely be a 0-10V output circuit.

If the voltage changes, you have a 4-20mA configuration.

I would like to remind you again that this simple test will cause disturbance to the control device. If you are not sure if that's OK, ask someone who knows.

Hi L.Chung,
Thank you,as you said, may I ask; if I remember correctly, Total current will increase with each parallel load added. If so, could this overload either of those circuit (4-10 or 0-10)?
Maybe I should find out how to make or understand each of these control circuits for how they do what they do?
Thank you for your time
Skinned

if I remember correctly, Total current will increase with each parallel load added. If so, could this overload either of those circuit (4-10 or 0-10)?

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For a 0-10V circuit, the output will be held fairly constant with different output loadings(within practical limits). That's why a 10KΩ resistor will not usually affect the output voltage as it draws only maximum 1mA.

For 4-20mA circuit, the 10KΩ is a high impedance and will thus taking less than 1mA current so there is no possibility of overloading the output.

The only concern is the slight disturbance when the 10KΩ is applied and removed.

BTW, why can't you find out properly via looking at drawings or asking your supervisor of the type of circuit you are working on? It is far much better than to have to do a test to find out what they really are.

Hi L.Chung,
Eblc1388 said (first answer to my question):If the voltage doesn't change with the press of the push button, then it would most likely be a 0-10V output circuit.If the voltage changes, you have a 4-20mA configuration.
May I ask; would it be ok to read the current for a 0-10vdc control circuit or read the voltage on a 4-20ma circuit? I have an old Beckman hd100 multi meter, should work fine. I have never tried that. Do you know what the outcomes would be?
You said:BTW, why can't you find out properly via looking at drawings or asking your supervisor of the type of circuit you are working on?
I am glad you asked, first of all, Cee-mens is a complicated company and if you dont work for them or dont buy millions dollars of their stuff, information dont come easy, very expensive, and its not given to those who want it even if you are the one buying it. This is happening everywhere now. For instance back in the 80s an electrical diagram would give you an understanding of how it works exactly, if you knew the basics. But now, wires go into a mystery box and on the other side of the box wires come out to contacts or what it controls! So, no Sir. Its very hard to do it the right way here in the USA!
My supervisors are overpaid sales persons who only know what something does not how its done! Its up to me to make it work.

Current loop sensors are typically the source, and subsequently powered. Voltage devices may be the source, but are often resistive in nature. Recievers on a current loop are often powered from the loop, hence only terminate on the loop.

If one works on such systems, they must know the mode of operation, including the input requirements of the reciever. PLCs typically have either a current or a voltage input module, or a multi terminal module, where the termination is made either on the voltage or current inputs as required. Many recievers can be soft programmed for the input mode, and can be viewed by stepping through the configuration menus.

GetDeviceInfoHowdy,Mode of operations, most of them are typical, there are so many! But dont know them all. As long as I understand the mode of what I am working on should be fine.Not getting involved in the programming or configuration menus of the PLC, but maybe an output or input device.For the inputs and output, thats why I am here trying to learn to read them safely.Sir, are you implying that without diagrams and manuals it cant be understood, or repaired? Thank youskinned

Hello L.chung, Thank you for your input its helpful.Yes I am looking for just common information, just some help. LOL, trade secrets maybe latter.Originally Posted by skinnedMay I ask; would it be ok to read the current for a 0-10vdc control circuit or read the voltage on a 4-20ma circuit?I asked this question, because if I dont know what kind of circuit it is, I have to put my multi meter on or in it (current or volts) to see what it is (4-20ma, or0-10V). You did say, I can do that, so could you tell me for a mA circuit, microvolts, millivolt, or volts? 0-10v circuit mA, or uA? I need to set my meter.Also if I used that 10,000 ohm resistor, that slight disturbance you were talking about, did you mean it will change the signal slightly or is there something else?Thank youskinned

Look at the PLC program and find the percentage output that your particular loop is operating at. Divide it by 10 to determine the PLC's output voltage if it was 0-10 volts. Measure the actual output and if it is markedly different, it is a 4-20 ma loop.

Sir, are you implying that without diagrams and manuals it cant be understood, or repaired?

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Yes, and No. I've experienced situations where a maintenance person has little skills, yet can keep an operation running by hit and miss methods. On the other hand, troubleshooting is a two part process, one part information gathering, another part information utilization. Effective repair can only come about through the understanding of the problem. For me, information leads to understanding.

You've queried if your control line is voltage or current, are you sure it's not digital? Why would it be either of these in it's application? Why do you need to know? If you suspect the device is problematic, isolate and test.
You may be able to discover the method by experimentation, but viewing a schematic would likely end your query without disruption.

Here's the deal, if you are working on someone's equipment, you should be in a training program, unless you are qualified. If your in a training program, don't lurk around being embarassed to ask questions, demand the right to learn and grow.

This circuit is voltage Circuit due to one basic reason as he is saying Voltage across is less than One volt.For 4-20mA circuit is mostly 8-36 volt range.So according to my knowledge Excitation voltage less than 1 volt very doubtful. Waiting for Experts answer maybe I am wrong.

Jaguarjoe said: Look at the PLC program and find the percentage output that your particular loop is operating at. Divide it by 10 to determine the PLC's output voltage if it was 0-10 volts. Measure the actual output and if it is markedly different, it is a 4-20 ma loop.

I was looking to find the answer by a multi meter.

Jaguarjoe said: This circuit is voltage Circuit due to one basic reason as he is saying Voltage across is less than One volt.For 4-20mA circuit is mostly 8-36 volt range.So according to my knowledge Excitation voltage less than 1 volt very doubtful. Waiting for Experts answer maybe I am wrong.

You think 8-36 vdc is typical for current circuit, which will make it easy to find then.
I checked a control circuit today that was a 2-18vdc and found it to operate from .08-1.0 mA.
Thank you
Skinned

Greetings,
“WOW”, so many questions. Everything that you have said is right, I can’t argue against your point on how to do perfect service and repairs. But in my world it’s not like that. Do I need to pass a test or something to ask a question?
I asked: I work in control systems sometimes and I was wondering if you come to a circuit and you know that it is most likely either 0-10vdc, or 4-20mA, how do you find out which one it is?Sir, I am asking you please, share some knowledge that’s all. I had a temperature problem that was off by80*. I isolated the thermistors and GotThermistorInfo J, but found thermistors ok. Found a bad connection crimp to athermistor. Replaced crimps and found operation normal. I did not use my meter, for was not sure what it was. So then I thought how can I check safely for the signal type. For right now dc and amps, next will be digital ok.You offered me a deal. Well yes I am in a training class but other. Yes I ask questions, sit at the front too.Thank you for your time sir!skinned

The application (thermistor) immediately points to a voltage signal. However, there is no reason to suggest that it follows a 0-10 v format, and likely doesn't. The sensor will be scaled within the controller.

4-20ma, 0-10v, etc, are process standards. If your measuring a thermistor, you likely have a dedicated sensor circuit, which is a different thing. There are no standards other than what the manufacturer of the controller calls for. The simplest test in this case would be to substitute the thermistor for a potentiometer to mimic full scale resistance, or, to measure the the thermistors resistance over a temp range and compare to datasheet. Some devices are linear, some not. Scaling is done in the controller.

'Voltage' as a descriptor must be taken lightly. The controller typically drives a calibrated current over the resistive device, and reads the resultant voltage. You should be able to read a voltage change with a change in the thermistors value.

However, on the output side of the controller, now your likely talking a process standard of 0-20ma, 4-20ma, 0-10v, etc.